RE: [sv-ec] member initialization at declaration vs. constructor - order issue

 
I would think that i is not initialized until tbe constructor has been
completed. So i = 0, j = 0.
AFter super.new(j), then i = 123
Literally the constructor creates the data structure for the class
variable.
So the members cannot be initialized until the data structure has been
created.
That is my reasoning for the behaviour.
I am not certain of the behaviour is i was a static member.


Francoise
    '
-----Original Message-----
From: owner-sv-ec@eda.org [mailto:owner-sv-ec@eda.org] On Behalf Of
Daniel Mlynek
Sent: Friday, August 29, 2008 6:24 AM
To: Steven Sharp
Cc: sv-ec@eda-stds.org
Subject: RE: [sv-ec] member initialization at declaration vs.
constructor - order issue

Similar problem :
class C;
    int i=123;
    function new();
        int j=i;  //this initialization is perofrmed before I
initalization at declaration?
        //super.new(j);//implicit or explicit What would be the value if
j according to yours undertanding? 0 or 123?
 

-----Original Message-----
From: owner-sv-ec@server.eda.org [mailto:owner-sv-ec@server.eda.org] On
Behalf Of Steven Sharp
Sent: 29 sierpnia 2008 00:15
To: sv-ec@server.eda-stds.org; daniel.mlynek@aldec.com
Subject: Re: [sv-ec] member initialization at declaration vs.
constructor - order issue


>From: "Daniel Mlynek" <daniel.mlynek@aldec.com>

>I've hit  another problem connected with order of execution of 
>declaration initialization vs.. constructor (imho allowing declaration 
>initialization for class members was good idea at all) LRM doesn't 
>define how it should be done. My guess is that:
>1st initialization from declaration should be done in order from base 
>class to child class 2nd constructor chain should be executed - as 
>described in LRM from base to child

The problem with this ordering is that an initializer on a declaration
in the child class could reference a property in the base class that is
initialized by the base class constructor.  The child class should not
be dependent on how the construction of the base class was done.
Therefore the base class should be completely initialized before the
initialization of the child class is started.


>But for classes inheriting some other class LRM says:
>"Every class has a default (built-in) new method. The default 
>constructor first calls its base class constructor (super.new() as 
>described in 8.14) and then proceeds to initialize each member of the 
>current object to its default (or uninitialized value)."
>Most interesting is "proceeds to initialize each member of the current 
>object to its default (or uninitialized value)." I thought that 
>uninitialized value is same as default, anyway why constructor need to 
>do some initialization if the value should be default (which means that

>value is hold in variable if no initialization or assignment was done
).
>But one can understand this sentence differently - default value is a 
>value from initialization at declaration - because it has to be 
>assigned

I believe this is how you must interpret the sentence, though I agree it
is not entirely clear.

The order should be

1. Call the base class constructor.
2. Initialize each member to the default value specified in its
declaration,
   if any, else to the default initialization value.
3. Execute the body of the constructor.

Though the LRM does not say so, it would also make sense to initialize
the properties in the order they were declared, since the initializer of
a property might refer to a property declared earlier.

This leaves only one small hole where you could refer to the value of a
property before it is initialized.  That is the one you use in your
example: the actual arguments to the super.new() call, if you specify it
explicitly.  In your example, you refer to a property of the local
class, which is not initialized until after the super.new() call.  You
could also refer to a property of the base class, which might be
initialized by the
super.new() call itself.  I think we just have to regard that as bad
code.

To make that code illegal, you would have to specify special scoping
rules to the actual arguments of the super.new() call.  You would have
to regard the scope the same way as if you were inside a static method
of the class:
you have access to the locals of the new() method and to static
properties, but not to nonstatic properties.  Another way to do it would
be to treat the value of 'this' as if it were null until after the
super.new() call returned.

>I totally don't understand what was the goal of adding this sentence to
LRM.

I suspect it was intended to answer your question about the order.  You
just have to interpret "default" in that sentence as meaning the value
on the declaration.  That would explain why the sentence treats it as
something different from the uninitialized value.
 

>What should be the results in your opinion? 

123 undefined 123 123

Making sure that the base class is initialized before the derived class
is more important than someone trying to pass an uninitialized property
as an argument to the base constructor.  Even with your suggested
ordering, I could still reference an uninitialized value in the base
class anyway.

With your suggested ordering, there would be a problem with

        class C;
                int i;
                function new(input int v);
                        i = v;
                        $display("C %d", i);
                        $display("C %d", v);
                endfunction
        endclass
 
        class CC extends C;
                int j=i;
                function new;
                        super.new(123);
                        $display("CC %d",i);
                        $display("CC %d",j);
                endfunction
        endclass

With your suggested ordering, the result of this would be

  123 123 123 undefined

With my suggested ordering, the result of this would be

  123 123 123 123

>Is the LRM description clear in this matter in your opinion?

It could use some improvement.


>Do you finding class member initialization at declaration useful 
>feature - or are you regretting that someone have vote it in into the
standard ?

It would certainly be simpler to have left it out and require users to
insert assignments into the constructor instead (after any super.new()
call).  But as long as it is understood to be equivalent to that, it
does not cause any real problems.


Steven Sharp
sharp@cadence.com


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